1. Field of the Invention
The disclosures herein relate to an imaging apparatus provided with a photoelectric conversion film and an electron source array having electron sources arranged in matrix form wherein electrons are emitted from the electron source array during a video signal output period and a vertical blanking period.
2. Description of the Related Art
Research has been conducted for some time with respect to imaging apparatus that is provided with a photoelectric conversion film and an electron emission array having a matrix of electron emission sources, from which electrons are drawn out by an electric field without application of heat. This electron emission array has a plurality of Spindt-type emitters arranged in matrix form, which are opposed to the photoelectric conversion film across vacuum space. In such imaging device, holes that are generated and accumulated in the photoelectric conversion film in response to light arriving from an external source are read out by using electrons successively emitted from the Spindt-type emitter array, thereby producing a time sequence of video signals (see Patent Document 1).
When highly bright light enters a portion of the photoelectric conversion film of such an imaging apparatus, a large amount of holes will be accumulated in this portion of the photoelectric conversion film on its electron scan side, resulting in a potential increase at this local position. As electrons are successively emitted from the electron emission array, those electrons emitted toward the vicinity of the portion of the photoelectric conversion film that is illuminated by highly bright light have their trajectory bent towards this portion of the photoelectric conversion film having an extremely high potential. This phenomenon will hereinafter be referred to as “bending”. As a result of the bending, holes accumulated in this portion of the photoelectric conversion film will be read out. Consequently, the output video image suffers an artifact by which a highly bright object appears expanded from its original size (which will hereinafter be referred to as “blooming”), degrading image quality such as image resolution.
Further, when highly bright light enters a portion of the photoelectric conversion film, a large amount of holes are generated and accumulated in this portion. When this happens, it may not be possible to read out all the generated and accumulated holes by use of the electrons emitted from the electron emission array on a single scan, thereby creating a prominent capacitive residual image.
Moreover, the entry of highly bright light into the photoelectric conversion film causes a large amount of holes to be accumulated on the electron scan side of the photoelectric conversion film. This serves to drop the effective electric field applied to the photoelectric conversion film, resulting in a large amount of optically generated electric charge (electrons and holes) being trapped in the photoelectric conversion film. As the electric field in the photoelectric conversion film increases in response to scanning on the photoelectric conversion film by electrons successively emitted from the electron emission array, the optically generated electric charge is released from its trapped state. Holes of this freed electric charge are then accumulated on the electron scan side of the photoelectric conversion film. As a result, these holes that were trapped are read out on a next scan, creating a prominent photoconductive residual image.
In order to obviate these problems, study has been conducted with respect to a flat-type imaging device that emits electrons in an excessively-accumulated charge sweeping period. This excessively-accumulated charge sweeping period is situated following a video signal output period, during which pixel signals on a horizontal scan line is read out (see Patent Document 2, for example).
This flat-type imaging device applies a voltage to the gate electrode of a next horizontal scan line in an excessively-accumulated charge sweeping period, and sets the potential of cathode electrodes higher than the potential of a reference scan surface. This serves to remove the accumulated electric charge that is provided in excess of the amount readable within the video signal output period. With this arrangement, white saturation, smear, and resolution degradation resulting from imaging a highly bright object are prevented.
In order to prevent white saturation, smear, and resolution degradation resulting from imaging a highly bright object, Patent Document 2 discloses applying a voltage to the gate electrode of a next following horizontal scan line and also setting the potential of cathode electrodes higher than the potential of a reference scan surface in an excessively-accumulated charge sweeping period following a video signal output period. The disclosed flat-type imaging device can thus remove the electric charge that is accumulated in excess of the amount readable during a video signal scan period.
In the imaging device disclosed in Patent Document 2, however, the amount of electrons used to remove excessively accumulated electric charge is limited to the amount of electrons that are emitted during a single excessively-accumulated charge sweeping period within one field or one frame. The effect of electric charge removal thus may not be sufficient depending on imaging conditions. Especially when highly bright light such as that of a street lamp or the headlights of a car enters a photoelectric conversion film during filming at night while the diaphragm of the optical lens is open, the amount of holes generated and accumulated in the photoelectric conversion film may be hundreds to thousands times greater than the amount of holes obtained under normal circumstances. It is thus not possible to remove all the excessive holes generated and accumulated in the photoelectric conversion film within a single excessively-accumulated charge sweeping period. In this case, image degradation is unavoidable due to the lowering of image resolution resulting from blooming and also due to the generation of capacitive residual images.
In the imaging device disclosed in Patent Document 2, also, the excessive holes accumulated in the photoelectric conversion film on the horizontal scan line (first horizontal scan line) from which a video signal will next be output is removed during an excessively-accumulated charge sweeping period. There may be a case in which a large amount of holes are accumulated on the next horizontal scan line (second horizontal scan line adjacent to the first scan line) from which a video signal will be output next following the first horizontal scan line. In such a case, the electrons successively output from the first horizontal scan line of the electron emission array to detect a video signal is bent towards the position of the second horizontal scan line of the photoelectric conversion film having a higher potential, thereby ending up reading holes accumulated there. This gives rise to a problem that blooming is generated, and image resolution is degraded.
Further, the imaging device disclosed in Patent Document 2 cannot prevent the generation of a photoconductive residual image resulting from the lowering of an electric field within the photoelectric conversion film in response to highly bright incident light. Especially when a highly bright object is in motion, the object leaves a visible trace on the screen. This causes a severe reduction in image quality.
Accordingly, there is a need for an imaging apparatus that can prevent the degradation of image resolution resulting from blooming occurring due to highly bright incident light, and can also prevent the generation of capacitive or photoconductive residual images.
[Patent Document 1] Japanese Patent Application Publication No. 6-176704
[Patent Document 2] Japanese Patent Application Publication No. 2004-134144